Potassium channels are ubiquitous proteins and their exceptional functional diversity makes them ideal candidates for a large number of biological processes. They intervene notably in the regulation of neuronal and muscular excitability, cardiac rhythm and hormone secretion. Three structural types of potassium channels have been described in mammals. The first is the Shaker type which is composed of subunits that have six transmembranal segments and one P domain which is implicated in the formation of the ionic pore. The second is the IRK type which has two transmembranal segments and one P domain. The third has been described more recently and corresponds to the TWIK type which has four transmembranal segments and two P domains. Three channels of this type have been identified: TWIK-1 (Fink, M. et al. EMBO J. 15, 6854-6862 [1996]; Lesage, F. et al. EMBO J. 15, 1004-1011 [1996]), TREK-1 and TASK (Duprat, F. et al. EMBO J. 16, 5464-5471 [1997]). In addition to a conserved general structure, they have primary sequences exhibiting little similarity since they present between 20 and 25% amino acid identity.